SUMMARY: Circulating high density lipoproteins (HDL) are highly heterogeneous from both a compositional and functional perspective. Currently, over 95 proteins are known to associate with plasma HDL and these drive a number of potentially cardioprotective functions. HDL-like particles also exist in the brain and cerebral spinal fluid (CSF) where they perform functions ranging from lipid transport to regulation of neurobehavioral function including roles in maintenance of body weight and energy balance and cognition. These lipoproteins tend to be dominated by apolipoproteins E and J and appear to exhibit distinct structures and compositions vs. their plasma counterparts. In this supplement request, we plan to test the hypothesis that apoE genotype (i.e. E3 vs E4) will significantly alter the lipoprotein size profile of CSF lipoproteins. Furthermore, we anticipate that the structure of these species, as determined by chemical cross-linking combined with mass spectrometry, will be significantly altered by apoE genotype. Working with a leading Alzheimer's Disease (AD) expert, we will apply a new technology that we developed in our laboratory to study the structure and composition in human CSF and relate that information to AD susceptibility. This method allows, for the first time, characterization of the size and composition of low abundance CSF using non-perturbing gel filtration separation. Although we have studied plasma lipoproteins in the past under the parent grant, this work will be a logical extension of our technology and expertise into the AD field. We anticipate that the proposed studies will provide foundational preliminary data for a future RO1 application targeting the structure and function of CSF lipoproteins in AD susceptible populations.